Cathode structure and electron gun for cathode ray tubes

ABSTRACT

The invention relates to a cathode structure intended to be inserted into an electron gun for a cathode-ray tube. 
     The cathode comprises a cathode body on the end of which is placed an emissive pellet, the cathode body being held in place inside a sheath by supporting means which comprise: 
     a first series of branches, each branch being connected on one side to the cathode body and on the other side to an intermediate piece 30; and 
     a second series of branches, each branch being connected on one side to the sheath and on the other side to the same intermediate piece.

The invention relates to a cathode structure intended to be insertedinto an electron gun for a cathode-ray tube.

The current trend is tending towards requiring cathode-ray tubes ofincreased performance in terms of screen brightness, lifetime, bright-uptime and consumption.

Most of these parameters essentially depend on the structure and thetype of cathode used to generate the electron beam or beams which willscan the screen of the tube.

The oxide-coated cathodes normally employed hitherto are at their limitswith regard to these requirements, and the tendency is to replace themwith dispenser cathodes which make it possible to achieve higher currentdensities with longer lifetimes.

Dispenser cathodes or impregnated cathodes operate at temperatures ofabout 1000° C.-1200° C. The expansions of the constituent materials ofthe cathode at these temperatures have to be minimized in order toobtain good performance stability of the electron gun into which thistype of cathode is inserted, this being achieved by using a refractorymaterial and dimensions of the cathode support which limit theconductive heat losses.

To achieve this result, Patents U.S. Pat. No. 4,184,100 and U.S. Pat.No. 5,218,263 illustrate two types of structure commonly employed:

a cathode body of substantially cylindrical shape having, at one end,the emissive part and containing the heating element;

a metal sheath which serves as shielding, by acting as a heat shield, ofsubstantially cylindrical shape surrounding the cathode body; and

a means for supporting the cathode body inside the shielding.

The supporting means must be such that they allow rigid assembly whilestill minimizing the conductive heat losses. The means for supportingthe cathode body may be tabs, produced from metal strips, of smallcross-section in order to minimize the thermal losses, the ends of thesetabs being connected on one side to the cathode body and on the otherside to the shielding. In another embodiment, the tabs are cut out fromthe cylindrical part of the shielding so that one end remains integralwith the shielding while the other end is connected to the cathode body.

However, the fact of using metal tabs of very small cross-section doesnot enable sufficient mechanical rigidity to be achieved, when fittingthe cathode body in its sheath and fixing it thereto; moreover, theseconnection tabs may be easily deformed by the thermal expansion of thecathode body during its operation.

Patent Application EP 534,842 describes a structure in which thesupporting means are more rigid, since they consist of a substantiallyfrusto conical ring whose surface is pierced with holes. However, thisstructure has the drawback of promoting heat exchange between thecathode body and its sheath, thereby impairing the thermal efficiency ofthe cathode.

An object of the present invention is to improve the cathode supportstructures in order to allow both rigid assembly of the cathode in itssupport and minimization of the conductive heat losses between thecathode body and its sheath.

To do this, according to one embodiment of the invention, the cathodestructure comprises means for supporting a cathode body inside a metalsheath, wherein the supporting means include a first series of branchesconnected to the metal sheath via one of their ends and a second seriesof branches connected to the cathode body via one of their ends, thesecond ends of the first and second series of branches being connectedtogether by an intermediate piece.

Other advantages will emerge from the description and from the drawings,among which:

FIGS. 1 and 2a and 2 b illustrate means for supporting cathode bodies intheir sheath, according to the prior art;

FIG. 3 shows, in an exploded view, a cathode structure in accordancewith the invention;

FIG. 4 shows, in a side view from above, a cathode structure accordingto another embodiment of the invention;

FIG. 5 shows an example of supporting means in accordance with theinvention, consisting of a single metal piece; and

FIG. 6 illustrates the mechanical stresses which the means forsupporting the cathode body, according to a first embodiment of theinvention, experience when the cathode is brought to temperature.

As illustrated in FIG. 1, an impregnated cathode 1 according to theprior art consists of a cylindrical cathode body 3 which comprises anemissive part 6 at one of its ends and, housed in the hollow part of thecathode body, a filament 5 for heating the emissive part. The cathodebody is connected to a hollow metal sheath 2 by means of the partialcut-out in the surface of the sheath of tabs 4, one end of which remainsintegral with the sheath while the other end is pushed towards theinside of the sheath and welded to the cathode body. In order to avoidheat losses from the cathode body to the sheath, it is necessary to usetabs of very small cross-section, something which is difficult toachieve here, the thickness and width of the tabs being limited by theminimum thickness of the sheath (approximately 25-30 μm), by the accesspoints for spot-welding the ends of the tabs to the cathode body and bythe difficulties of cutting out from these materials.

FIGS. 2a and 2 b illustrate another embodiment, seen from above and incross-section, according to the prior art. The cathode 11 has a cathodebody 13 which includes an emissive part 16 and a heating filament 15,the body 13 being inserted into the sheath 12 and connected to thelatter via supporting means 14 in the form of a crown of substantiallyfrusto conical shape and drilled with holes 17. These supporting meanshave the advantage of providing sufficient mechanical rigidity in orderto hold the cathode body in place in its sheath, but has the drawback ofpromoting heat exchange between the body 13 and its sheath 12 because ofthe large quantity of material forming bridges between the pieces 12 and13.

As illustrated in FIG. 3, a cathode 21 in accordance with the inventioncomprises a cathode body 23 held in position inside the sheath 22, withthe aid of the following means:

a first series of branches 31, each branch being connected, on one side,to the cathode body and, on the other side, to an intermediate piece 30;

a second series of branches 32, each branch 32 being connected, on oneside, to the sheath 22 and, on the other side, to the same intermediatepiece 30.

The branches 31 and 32 may be connected by welding to the piece 30 ormay form with it a single piece produced, for example, by cutting outfrom a metal plate, thereby decreasing, in the latter case, themanufacturing cost of the cathode body support.

The intermediate piece 30 is preferably chosen in the form of a ring,the diameter of which has a value intermediate between the internaldiameter of the sheath and the external diameter of the cathode body.The mechanical connection between the two series of branches stiffensthe cathode support and enables branches of small cross-section to beused, these offering the advantage of low heat dissipation.

In one embodiment of the invention, as illustrated in FIGS. 3 and 4, thefirst series of branches 31, one end of which is welded to the sheath22, comprises three branches arranged at 120° with respect to eachother; the second series of branches 32, one end of which is welded tothe cathode body supporting the emissive part 26, also comprises threebranches arranged at 120° with respect to each other. This structureenables the number of heat bridges between the cathode body and itssheath to be minimized, while at the same time providing good mechanicalstability and good positional retention of the cathode body. However,this number is not restrictive, it being possible to use more than threebranches in each of the series of branches and/or a different number ofbranches in the first series of branches from the second.

In a preferred embodiment of the invention, those parts of a series ofbranches connecting the intermediate piece 30 either to the cathode bodyor to the sheath 22 lie in the same radial plane perpendicular to theaxis Z of revolution of the cathode. The result obtained is that themeans for connecting the cathode body to its sheath are shortened andthat it is possible to obtain a shorter cathode structure than in theprior art. Those parts of the two series of branches 31 and 32connecting the intermediate piece 30 to the cathode body and to itssheath may advantageously lie in the same radial plane perpendicular tothe axis Z of revolution of the cathode. This structure has theadvantage of further shortening the total length of the cathode and ofallowing the use of a shorter filament 25 heating the emissive body 26than in the prior art, resulting in a very rapid bright-up time becauseof the low thermal mass involved.

In another embodiment, the support for the cathode body, illustrated inFIG. 5, comprises a first series of branches 31 extending outwards fromthe ring starting from junction points 34 and a second series ofbranches 32 extending inwards from the ring starting from junctionpoints 33, the junction points 33 and 34 being offset so as not to bearranged opposite each other.

The means 30, 31, 32, for supporting a cathode, comprising a cathodebody 1.8 mm in diameter and a sheath 4.6 mm in diameter, may be producedby cutting out from a sheet of metal, 25 μm in thickness, for examplenickelchrome in respect of an oxide-coated cathode ortantalum-impregnated cathode. The width of the branches 31, 32 and ofthe ring 30 is 0.4 mm, and the mean diameter of the ring is 3.2 mm. Eachbranch 31 includes a first part 35 intended to be placed between thering 30 and the sheath 22, and an end portion 135 which, once folded sothat it is brought parallel to the internal surface of the sheath, willallow the branch to be welded to the sheath. Likewise, each branch 32includes a first part 36 intended to be placed between the ring 30 andthe cathode body 23, and an end portion 136 which, once folded so as tobe brought parallel to the external surface of the cathode body, willallow the branch to be welded to the cathode body.

If it is chosen to use N branches 31 offset with respect to each otherby an angle θ=2π/N, N branches 32 are also used, the junction points 33of the branches 32 being offset by θ/2 with respect to the junctionpoints 34 of the branches 31. This supporting-means structure has theadvantage of increasing the length of the thermal linkage between thecathode body and the sheath, thereby increasing the temperature gradientbetween the said cathode body and the sheath, of decreasing theconductive heat losses and of shortening the bright-up time of thecathode. Moreover, the offset between the junction points of theinternal branches 32 and external branches 31 with the intermediatepiece 30 makes it possible to carry out the welding operations moreeasily, in two steps, firstly the internal branches to the cathode bodyand then the external branches to the sheath 22. The welding may, inthis case, be carried out by laser welding or by electric resistancewelding by virtue of the clear space obtained.

In another embodiment derived from the previous one, the ring 30 and theparts 35 and 36 of the branches 31 and 32, respectively, lie in the sameplane, perpendicular to the axis Z of revolution of the cathode body.This arrangement increases the mechanical stability of the cathodestructure; in particular, it prevents movements of the cathode bodyalong the radial axis Z which are due to the elongation of the supportbranches 31 and 32 by thermal expansion during operation of the cathode.In this case, as indicated in FIG. 6, when the cathode is operated attemperature, the parts 35 and 36 of the branches for supporting thecathode body will be able to lengthen in the radial direction, due tothe effect of the temperature, exerting respectively centrifugal forcesF and centripetal forces FR on the ring 30, the ring 30 undergoingelastic deformation in the radial plane, under the effect of theseforces, as indicated by the dotted lines in FIG. 6. Since the mechanicalstresses due to the elongation of the suspension branches are thusabsorbed by the ring, the position of the cathode body with respect toits sheath will not vary during the transient cathode-heating period.

As already indicated in respect of the other embodiments of the presentinvention, the structure described by FIG. 6 is equally applicable tothe production of an oxide-coated cathode as to impregnated cathodes.

In an embodiment not shown, the supporting means include an intermediatepiece 30 made from a different metal from the branches 31,32, thebranches being fastened to the intermediate piece, for example by laserwelding. The advantage of this structure is that it allows the metal ofthe branches 31, 32 to be chosen depending on its abilities to reducethe thermal conduction and enables the metal of the intermediate pieceto be chosen depending on its mechanical elasticity properties.

What is claimed is:
 1. Cathode structure for a cathode-ray tube, whichcomprises means for supporting a cathode body inside a metal sheath, thesupporting means including a first series of branches connected to themetal sheath via one of their ends and a second series of branchesconnected to the cathode body via one of their ends wherein the secondends of the first and second series of branches are connected togetherby an intermediate piece.
 2. Cathode structure according to claim 1,wherein the intermediate piece is in the form of a ring.
 3. Cathodestructure according to claim 1, wherein the branches of at least oneseries are arranged over part of their length in the same plane. 4.Cathode structure according to claim 2, wherein the intermediate pieceis in the form of a flat ring and in that the branches of at least oneseries are arranged over part of their length in the same plane as theplane of the ring.
 5. Cathode structure according to claim 1, whereinthe intermediate piece, the first series of branches and the secondseries of branches form a piece made as a single component.
 6. Cathodestructure according to claim 1, wherein the first series of branchesand/or the second series of branches have at least three branches. 7.Cathode structure according to claim 1, wherein the junction pointswhere the branches of the first series of branches join the intermediatepiece are offset with respect to the junction points where the branchesof the second series of branches join the intermediate piece.
 8. Cathodestructure according to claim 7, wherein the first and second series ofbranches have the same number of branches and in that the junction pointwhere each branch of the same series joins the intermediate piece isarranged half-way between the junction points where the two consecutivebranches of the other series join the ring.
 9. Electron gunincorporating a cathode whose structure is according to claim
 1. 10.Cathode-ray tube incorporating at least one electron gun according toclaim 9.